This invention relates generally to road construction and asphalt paving machines, and relates more particularly to non-contacting sensing and control devices for controlling the position of a grading implement relative to a datum. Further, this invention particularly relates to controlling the rotational velocity of an auger and the position of a gate to a hopper relative to the profile of asphalt on the ground.
As described in U.S. Pat. No. 4,733,359, motor graders, bulldozers, pavers and other road construction earth moving vehicles have been used for controlling the position of a grading implement relative to a datum. The grading implement of these vehicles are usually positioned by hydraulic cylinders that are coupled to mechanisms that support the grading implements. As the paving machine moves along the road bed, a mechanical sensing device or range sensing device senses the position of the datum relative to the implement, and a control device then signals the hydraulic cylinders to reposition the implement accordingly.
As is known in the art, an implement used with a paving machine to level the asphalt is a screed. The screed is positioned by a hydraulic cylinder responding to a signal from a control and sensing device. Two types of sensing devices for adjusting the position of a screed include a contacting wand and a non-contacting sonic sensor. The wand drags along the ground and swings upward or downward depending on the level of the datum. This wand may go off the line of the datum and also may become jammed. An improvement to the contacting wand is a non-contacting sonic device as described in U.S. Pat. No. 4,733,355. The non-contacting sonic device has been found to be more reliable than the contacting sensing device as there are few movable parts and is less susceptible to errors due to mechanical failures.
Sonic devices have a reference wire located a known fixed distance down from the sonic sensor. The sonic sensor transmits a sonic signal which reflects from the reference wire. This time between transmission and receipt of the sonic signal from the reference wire is used to adjust the position of the screed.
When using a non-contacting sonar device with a reference wire, it is possible to have a screed that levels pavement to a target level with an accuracy of less than one-tenth of an inch. Consequently, the screed placement is more precise, resulting in instantaneous screed movement which may cause small ripples in the road surface. It is therefore necessary to have a screed that travels along the road that provides a smooth road surface without ripples. Another drawback of the non-contacting sonic device with a reference wire is that during operation as air temperature varies, the screed position also varies. This may lead to the position of the screed deviating from the target level which may result in an uneven road surface. A third drawback of using a more accurate sonic sensor is that due to the instantaneous response to the changes in the road condition, the screed is apt to move up or down and miss the target level. A fourth drawback is that the sonic device may cause the screed to position itself in response to detection of a pot hole.
When using a paving machine, the asphalt is typically deposited into a hopper from which the asphalt is carried on a conveyer belt to the back of the paving machine. The asphalt then exits through the back of a paving machine into the center of an auger. The auger rotates and distributes the asphalt uniformly to the sides of the road bank where the asphalt is then flattened by the screed.
The auger typically runs at a variable rate or alternately may rotate at a fixed speed. The speed of the auger is controlled by a wand or sonic sensor device that resides near the outer ends of the auger. The wand senses the height of the asphalt by rotating upward as the asphalt level gets higher. The sonic sensor device senses the height of the asphalt by transmitting a sonic signal and noting the reflection time. The wand and sonic sensor device are connected to a potentiometer which changes its resistance as the wand extends outward or the reflection time becomes lower in response to a rise in the level of the asphalt. This resistance change causes the speed of the auger to change, resulting in an even level of asphalt being distributed laterally by the auger.
A first drawback of the mechanical control wand is that the arm resides in the asphalt and accordingly, asphalt sticks to the wand, thereby changing its center of gravity. This change of wand center of gravity may cause the arm to be stuck at a particular angle, thereby clogging and resulting in an inaccurate auger speed. This inaccurate speed results in the asphalt being distributed unevenly to the screed resulting in a build-up or a depletion of asphalt along the sides of the road bed. Further, because the arm is mechanical, the auger control does not respond to a change in the auger height at a fast enough rate, compounding the unevenness of the pavement to the screed. A second drawback of the mechanical wand and auger sonic sensor is that they may cause the auger rotational speed to fluctuate continuously because the height of of the asphalt within the auger is uneven. This unevenness results in a separation in the pavement material and a poor quality road. A third drawback to using a mechanical wand and an auger sonic sensor is that an inaccurate auger speed may result due to improper placement of the wand or the sonic sensor. A fourth drawback is that the auger may break down and jam due to continuous contact with the pavement material.
A paving device typically has a hopper gate to control the flow of the asphalt being distributed to the auger. This gate is currently regulated manually by the operator. The height of the hopper gate is important, as it further controls the amount of asphalt being distributed to the auger. When too much asphalt is delivered to the auger, a peak will form in the center of the paving machine, causing an uneven flow to the screed. If the gate is lowered, resulting in too little asphalt being distributed to the auger, the result is a trough in the center of the paving machine and excess asphalt being delivered to the screed along the sides of the road bed. To correct this problem, the operator must manually adjust the gate height; however, this still results in inaccuracies along the road bed, as the operator is not always able to react to changing road conditions.
A further drawback of using a wand or sonic device as screed sensing devices is that substantial time is required for set up and adjustment. Typically, a wand or sonic sensor is positioned on the paving machine at a predetermined height above the ground relative to the target level that the grader is to be positioned above the ground. Often between days of operation, these sensor devices are moved or bumped. Accordingly, the following day these control devices must be checked and reset again. Further, in operation, it is often required to change the height of the asphalt relative to the level of the grader, thereby requiring that the sensing device be moved to another position.